Bacterial endophyte antagonism toward a fungal pathogen in vitro does not predict protection in live plant tissue

Authors: WHITAKER, BRIANA - Indiana University; Bakker, Matthew

Submitted to: FEMS Microbiology Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/16/2018
Publication Date: 12/18/2018
Citation: Whitaker, B.K., Bakker, M.G. 2018. Bacterial endophyte antagonism toward a fungal pathogen in vitro does not predict protection in live plant tissue. FEMS Microbiology Ecology. https://doi.org/10.1093/femsec/fiy237.
DOI: https://doi.org/10.1093/femsec/fiy237

Interpretive Summary: The fungus Fusarium graminearum causes a damaging disease on wheat and other small grain crops, which not only reduces yield but also causes the accumulation of toxic compounds in the harvested grain. Other beneficial microorganisms also inhabit wheat plants and can slow down development of disease caused by Fusarium. In this work, we collected several hundred cultures of bacteria from various wheat tissues. For a subset of these bacteria, we tested how they interact with Fusarium, using a series of experiments that began under simplified conditions and progressed toward increasingly realistic conditions. Many of the bacteria that we tested were able to reduce the growth of Fusarium, but results from our simplified experimental systems did not match the results from the more realistic experimental systems. Knowing this, future studies of beneficial bacteria as antagonists of Fusarium will be performed under the more realistic conditions, which should improve the effectiveness of these studies. Within each experiment, we also tested bacteria-Fusarium interactions at different temperatures and under different atmospheric carbon dioxide concentrations. Together these experiments indicate that the ability of beneficial bacteria to control Fusarium head blight of wheat varies with environmental conditions such as temperature and carbon dioxide concentration. This indicates that any promising beneficial bacteria must be tested under a wide range of conditions to determine whether the beneficial activity occurs consistently.

Technical Abstract: Endophytic microbiota are potentially useful plant symbionts for conferring biotic or abiotic stress tolerance. Common approaches to identify putatively beneficial functions of endophytes rely on lab-based assays. However, if functional roles are context-dependent, lab-based assessments may not accurately represent functional outcomes under variable field conditions. Our objective was to test whether antagonism by bacterial endophytes towards a plant pathogen in vitro would be predictive of disease outcomes in live plant tissue. We challenged Fusarium graminearum, a fungal pathogen of wheat, against bacterial endophytes isolated from wheat plants in two in vitro assays. A subset of isolates, with in vitro antagonistic activity ranging from weak to strong, was selected for testing in live plant tissue (detached wheat heads). Assays were performed under different temperature and/or carbon dioxide conditions to test environmental dependency in the plant-endophyte-pathogen interactions. The two in vitro assays produced contrasting measures of pathogen inhibition, and neither predicted pathogen load reductions in the detached wheat head assay. Additionally, outcomes were environment-dependent and varied among bacterial isolates. Thus, endophytic impacts on plant performance cannot be easily inferred from simplified in vitro assays, and environmental gradients should be incorporated into future testing of microbial interactions in plant hosts.